1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus and, in particular, to a configuration for separating sheets one by one.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a facsimile machine, a copying machine, and a laser-beam printer includes a sheet feeding apparatus for feeding sheets such as plain paper, coated paper, plastic sheet and cloth to an image forming unit in the image forming apparatus.
It is very important for the sheet feeding apparatus to separate and feed sheets one by one to the image forming unit. In order to prevent the sheet feeding apparatus from feeding a plurality of sheets at the same time (double feeding), various types of feeding methods have been proposed.
In recent years, importance has been attached not only to stable feeding by preventing the double feeding, but also to downsizing of the sheet feeding apparatus. Furthermore, the image forming unit requires to be further downsized as a printer and a facsimile machine are used in a family.
Some image forming apparatuses including a sheet feeding cassette or a sheet feeding tray for storing sheets are provided with a feeding roller capable of forwardly or reversely rotating at the upper part of the sheet feeding tray, for example, to downsize the image forming apparatus.
When a sheet is fed, the sheet feeding apparatus reversely rotates the feeding roller to reversely transport the sheet in the sheet feeding tray and causes the trailing edge of the sheet to abut on the rear wall of the sheet feeding tray, thereby temporarily curving the uppermost sheet to be separated from other sheets.
After the uppermost sheet is temporarily curved, the feeding roller is forwardly rotated to send the uppermost sheet along the upper face of a separation claw provided downstream in the sheet feeding direction in the sheet feeding tray, thereby separating and feeding sheets one by one. Such a configuration allows the function of separating and feeding sheets to fall within the range where the sheet feeding tray is attached as discussed in Japanese Patent Application Laid-Open No. 05-147752.
In recent years, it has become important to stably feed various types of paper different in thickness. It often becomes problematic to feed a thin sheet which is low in stiffness in particular. A method for increasing apparent stiffness of a sheet by curving a sheet in a sheet feeding cassette, for example, to feed such a thin sheet low in stiffness.
As such a method, there is a method in which a sheet stored in a sheet support portion for stacking sheets, for example, is curved in the direction orthogonal to the feed direction of the sheet to increase apparent stiffness with respect to the feed direction of the sheet.
If the sheet is loosened by a sloping surface, increasing the apparent stiffness allows the sheet to be surely loosened even if the sheet is thin and the double feeding to be prevented. The technique is discussed in Japanese Patent Application Laid-Open No. 2000-143002.
In such a conventional sheet feeding apparatus, in a method for separating sheets in such a manner that the sheet is caused to climb up the separation claw, however, for a thin sheet low in stiffness in particular, when the sheet climbs up the separation claw, the leading end of the sheet collides with the edge face of the separation claw and may be broken. For a sheet high in stiffness, the leading end of the sheet catches the separation claw, resultantly, cannot climb up the separation claw, and may be jammed.
Then, as described above, the apparent stiffness of the sheet is increased in such a manner that the sheet is curved in the direction orthogonal to the feed direction of the sheet to allow preventing the leading end of the sheet from being broken, and allow the sheet to easily pass on the separation claw.
When the sheet high in apparent stiffness is curved by rotating the feeding roller in the direction opposite to the feed direction, the feeding roller may slip when the curve produced by reversely transporting the sheet is formed not to allow the sheet to be reversely transported.
When the uppermost sheet is curved by rotating the feeding roller in the direction opposite to the feed direction to cause the trailing edge of the sheet to abut on the rear wall of the sheet feeding tray, a buckle may occur at the trailing edge of the sheet because the sheet is high in apparent stiffness, which may not form warping in the sheet.